The abdominal skin of the frog, a model for the mammalian kidney, carries out transpithelial active sodium transport. Despite much study, functional parameters are not well defined and it is unknown whether the sodium pump responds directly to the inner membrane voltage Vi, the cell sodium activity acNa, or both. These problems will be approached by dynamic electrophysiological studies, altering the rate of transport by perturbing the transepithelial voltage Vt or external Na concentration. In addition to standard voltage clamp techniques for characterization of currents and conductance the study will employ conventional microelectrodes for measurement of outer and inner membrane voltage Vo and Vi, and neutral carrier membrane electrodes for measurement of AcNa and, where appropriate, aH and aCa. Following baseline observations, suitably rapid and brief exposure to amiloride should virtually abolish transcellular Na transport without significant secondary effects on paracellular conductance gp, thereby permitting its quantification. Prompt observations will then characterize the dynamic response of amiloride-sensitive cellular current flow Ia and conductance ga, Vo and Vi, the voltage divider ratio Fo = DeltaVo/DeltaVt, and (in tissues with minimal leak) the membrane conductances go and gi. Brief perturbations of Vt according to different schedules will define the protocol-dependence of parameter values. Longer perturbations will permit monitoring Ia as it evolves into the amiloride-sensitive cellular Na current, as well as the time course of Vo and acNa, the comparison of transient and steady state conductances, g and G, and in appropriate tissues, the evolution of the reversal voltage into the electromotive force of Na transport, ENa. Relating Ia and IaNa to acNa and Vi should demonstrate the determinants of pump function. Observations will also be interpreted in terms of a simple equivalent circuit, comprising an apical Na conductance, with a basal-lateral electrogenic Na-K pump (ATPase) in parallel with a passive K conductance. This should characterize fundamental parameters of the permeability barriers and the pump, and thus help to elucidate regulation of Na transport in the control state and in the presence of drugs and hormones.